Today, at Business Insider's Social Media ROI conference in New York, we'll tell you just how much advertiser money is pouring into social media, and which advertisers are making it pay off in sales.

But to set the tone, it's worth knowing who the biggest players are in the space. Unfortunately, neither Nielsen nor Kantar track display ad spending in social media, so figuring out who the whales are is tricky.

So, we asked our friends at ComScore to rank companies based on total impressions served, from January through August of 2012. We know that impressions served come at all different price points, but they're the best proxy we have for overall economic activity in the space. ComScore measured ads served on Facebook, Linkedin, Twitter, Tumblr, MySpace, Pinterest, DeviantArt, and similar sites (but no blogs).

The results — which form our Social Media Power List 2012 — are fascinating, especially when compared to our ranking of Facebook's biggest advertisers. Sure, some of the same players are here. But a lot of companies are apparently spending big off of Facebook too. Each slide in this gallery ranks brands by impressions served, and describes that client's overall social media marketing strategy.

Here's the most surprising result of ComScore's research: The No.1 company in social media buys more than three times the number of impressions as the second biggest client. Try and guess which company it is.